1) Field of the Invention
This invention relates generally to the removal of photoresist or stripping of photoresist in semiconductor manufacturing and more particularly to removal of photoresist residue from a keyhole in a passivation layer before a heating process using a stripping solution preferably comprising N-methyl-2-pyrrolidone (NMP).
2) Description of the Prior Art
The fabrication of integrated circuits and other devices on semiconductor wafers depends on the photolithographic patterning of successive layers of materials applied on and into the wafer. In each photolithographic step, a layer of photoresist is applied to the wafer, soft baked, and patterned by exposure to radiation through a precisely aligned mask. Such exposure alters the solubility of the resist material in a particular solvent, thus allowing selective removal of the resist in accordance with the pattern deemed by the mask. In this way, a precisely patterned protective layer is formed over the semiconductor wafer to limit subsequent fabrication steps to exposed locations on the wafer.
When the photoresist material becomes highly cross-linked, it begins to behave as a gel. When the cross-linking approaches 100% (unity), the resist is no longer soluble in any solvent and instead will only swell when exposed to even the strongest solvent for the starting polymer material.
In addition to cross-linking, contamination of a photoresist layer during a wafer fabrication step can also reduce its solubility. For example, when photoresist is used for patterning a silicon dioxide or aluminum layer during plasma etching, the organic photoresist material may become contaminated by silicon, aluminum, or other inorganic material. Such contaminated photoresists are frequently refractory to normal solvent removal.
Alternative removal techniques for refractory photoresists include several rigorous and often harsh processes. For example, thermal and photochemical oxidation of the photoresists have been employed. While generally effective, to assure sufficiently rapid processing, such oxidation requires elevated temperatures, typically in the range from 150.degree. C. to 250.degree. C. for photochemical oxidation and 250.degree. C. to 300.degree. C. for thermal oxidation. Such high temperatures can cause undesired diffusion within the semiconductor wafer, particularly resulting in diffusion of photoresist impurities into the device structures. Such high temperatures are unacceptable in many of today's integrated circuit manufacturing processes.
Removal of insoluble photoresist at lower temperatures, often below 100.degree. C., can be achieved by "ashing" in an oxygen plasma. The plasma discharge required to effect such ashing, however, can itself result in damage to the wafer substrate. While attempts have been made to limit such damage by separating the plasma generation chamber from the treatment chamber, photoresist removal efficiency is significantly lowered with such designs. To compensate for such lower efficiency, the wafer temperature is often increased, again raising concerns over thermal damage.
Wet oxidative stripping of insoluble photoresists is also performed, typically using a sulfuric acid-hydrogen peroxide mixture referred to as a "piranha strip." Such wet stripping techniques, however, are generally ineffective at moderate temperatures and often require temperatures above 150.degree. C. Frequently, wet stripping is performed only after an initial plasma ashing used to break through the upper region of the photoresist layer. In this way, the damage from the plasma can be minimized and the maximum temperature used during the wet stripping reduced. Such a combination of techniques, however, requires the use of separate reactors, is still rather slow, and uses relatively large amounts of wet chemicals.
For these reasons, it would be desirable to provide improved methods for removing (highly cross-linked) photoresist layers from semiconductor wafers. Such removal methods should be rapid. The methods should also reduce the amount of treatment chemicals required, both to decrease the cost of the process and to reduce contamination generated by the process.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the literature can be gleaned by considering: Moreau, Semiconductor Lithography, Plenum Press. 1988 .COPYRGT. pp. 779-788, which describes photoresist stripping processes.
U.S. Pat. No. 5,201,960 (Starov) shows a method for removing photoresist and other adherent materials from substrates.
U.S. Pat. No. 5,362,608 (Flaim) Microlithographic substrate cleaning and compositions therefor--teaches using THFA (tetrahydrofurfuryl alcohol) as the active component of edge bead removal compositions.
U.S. Pat. No. 4,786,578 (Neisius) teaches that photoresist and stripper residues can be completely removed from substrates after the stripping process with an aqueous post-rinsing agent which contains a nonionogenic surfactant and an organic base.
U.S. Pat. No. 4,824,762 (Kobayashi) shows a post strip rinse and method. Kobayashi suggests that further advantages are obtained by adding an aliphatic amine compound to the rinse solvent.
U.S. Pat. No. 3,988,256 (Vandermey)--Photoresist stripper rinse--shows a process for removing a photoresist stripper from a substrate comprising rinsing the substrate.